fix(treesitter): suppress get_parser warnings via opts.error

test/functional/lua/deprecated_spec.lua

@@ -0,0 +1,22 @@
+local t = require('test.testutil')
+local n = require('test.functional.testnvim')()
+
+local clear = n.clear
+local exec_lua = n.exec_lua
+local eq = t.eq
+
+describe('deprecated lua code', function()
+  before_each(clear)
+
+  describe('vim.treesitter.get_parser()', function()
+    it('returns nil for versions >= 0.12', function()
+      local result = exec_lua(function()
+        if vim.version.ge(vim.version(), '0.12') then
+          return vim.treesitter.get_parser(0, 'borklang')
+        end
+        return nil
+      end)
+      eq(nil, result)
+    end)
+  end)
+end)